This invention relates to the visual display of digitally compressed waveforms. More particularly, this invention relates to methods and apparatus for increasing on such a display the quantity of information representative of a compressed digital waveform mapped with respect to an independent variable, by providing intensity, color or other visually gradable features to indicate local change in the digital value of the waveform with respect to the independent variable.
In the use of devices for the display of digital data, such as digital oscilloscopes, it is often desired that a "macro" view of the data be provided, that is, long record lengths of compressed data are displayed in their entirety in order to aid in the visualization of the data as a whole, and to facilitate movement therethrough, toward a specific point of interest. Digital storage oscilloscopes ("DSOs") are increasingly being provided with large memories for acquiring large amounts of data to be so viewed. However, it is difficult to discern detail within a display of large amounts of data that are mapped onto a small number of screen widths.
One approach to this problem is to display a selected number of data points comprising a desired compression interval, usually the minimum and maximum values of the signal level therein, and consequently to discard information, with the effect of discarding information being known as decimation. In a columnar display, data within a compression interval produces a vertical bar, representing a vector having endpoints at the minimum and maximum values of signal level within the compression interval. Similarly, other columns representing other compression intervals are filled with vectors drawn between their associated minimum and maximum signal levels and thereby produce a band structure having an envelope of these values. This envelope represents all the information displayed.
Another approach to the problem of displaying a large amount of data in a relatively small screen width that has been utilized for the display of waveforms as a function of either time or frequency is to form a histogram of the number of times (hereinafter referred to as "hits") a digital signal within a given compression interval is acquired at a particular level (hereinafter referred to as a "histogram bin"). The data within the compression interval is then mapped entirely to a quantum of one of the axes of the display, usually a column of one pixel width, for the display of compressed waveform signal levels as a function of time or frequency.
One approach to performing the aforementioned mapping is exemplified by Bales, et. al., U.S. Pat. No. 4,890,237, hereinafter incorporated by reference in its entirety. In Bales, et al., discrete values of data within the compression interval are represented by corresponding points on the columnar display having a brightness proportional to the number of hits. While such a display provides more information than a columnar display of a vector representing only the minimum and maximum values over an interval, a number of unhelpful or misleading visual cues result from this mapping that may be appreciated by consideration of the display of a sine wave. If samples of a sine wave are acquired at a frequency that perfectly and evenly divides the frequency of the sine wave, the compressed result will be a column of bright spots at the sample values and darkness therebetween. The spacing of these spots will provide some visual indication of the slew rate of the sine wave, though not a particularly good one for either very low sampling rates, where there may be only a few spots representative of the period of the sine wave, or very high sampling rates, where the space between dots becomes less discernible. Further, if the acquisition frequency above differs slightly from a perfect and evenly dividing value, a moire pattern will result when adjacent columns representing other compression intervals are displayed. This pattern will tend to obliterate even the limited visual cues available with a perfect sampling.
Analog oscilloscopes have also been used, and are often preferred, for compressing and viewing long waveforms, because their method of operation inherently provides for the display of desirable information not inherently displayed by conventional DSOs. When the time-base on an analog scope is increased--meaning that the time axis displays a longer time--a waveform is "compressed" by its method of operation, that is, the horizontal or time resolution as a function of the speed of the ramp utilized for horizontal deflection of the CRT beam is limited. The resolution on the time axis may be thought of as a compression interval analogous (though continuous; not quantized) to that described above for a DSO. Though the time axis changes for data slew rates higher than the resolution of the time axis are visually undiscernible, intensity gradations within a column corresponding to the compression interval remain discernible. Signals that dwell on or about a given signal level within the compression interval keep the electron beam dwelling on the same location of the screen and so increase the perceived brightness of phosphorescence at that location by targeting those phosphors with greater frequency. Consequently, within a display column corresponding to a compression interval in an analog oscilloscope, values of the signal at which the signal dwells or from which the signal slews more slowly will be brighter than values at which the signal spends less time and from which the signal transitions more quickly.
DSOs have well-known advantages over their analog counterparts that makes their use preferable for many applications. For example, DSOs have the ability to postprocess, save, colorize, take many independent views of acquired data, and create persisting displays of a "single shot" of data. Therefore, it would be desirable to provide a DSO which has the intensity variation advantages of analog oscilloscopes.